1. Field of the Invention
The present invention relates to a fuel vapor control system of an internal combustion engine, and more particularly to a fuel vapor control system of an internal combustion engine which provides for accurate estimation of the fuel vapor amount purged or pulled from the charcoal of canister to the engine's air intake system and accurate correction of the fuel injection amount based on the estimated purged amount.
2. Description of the Related Art
It has been known to equip a charcoal canister in an internal combustion engine, filled with activated charcoal which absorbs and stores a large amount of fuel vapors leaving from a fuel tank through a vapor line (hose). The canister is connected to the engine air intake system (intake manifold) through a canister purge line (hose). When a purge solenoid valve equipped at the canister purge line is opened under predetermined engine operating conditions, the gasoline vapors stored in the canister charcoal bed are pulled into the air intake system and then into the combustion chamber for burning. Since the purged gasoline vapors disturb the engine air/fuel ratio control or fuel metering control, it has been desired to estimate the fuel vapor amount accurately and correct the fuel injection amount by the estimated amount.
Japanese Laid-open Patent Application No.6(1994)-26410 teaches calculating a current in-canister fuel vapor weight WC by subtracting a purged fuel vapor weight Fevpex from the in-canister fuel vapor weight calculated up to the last calculation cycle and by adding an in-canister-absorbed fuel vapor amount Fevpin. In the reference, the current in-canister fuel vapor weight WC thus calculated is then multiplied by a purge flow rate correction coefficient KQevp and another correction coefficient .alpha.evp (determined based on an O.sub.2 air/fuel ratio feedback correction coefficient .alpha.) to determine a desorption factor KFevp.
In the reference, the desorption factor KFevp is then multiplied by a purge flow rate Qevp and a fuel specific weight .gamma.evp to determine a purged fuel vapor weight Fevpex. The purged fuel vapor weight Fevpex is then multiplied by an injector correction coefficient Kinj to convert the same into a pulse-width correction coefficient (correctional purged fuel amount) Tevp. The pulse-width correction coefficient Tevp is then subtracted from a basic injection pulse-width (corresponding to a basic fuel injection amount). The difference is then multiplied by the O.sub.2 air/fuel ratio feedback correction coefficient .alpha. and similar parameters to finally determine an output injection pulse width (corresponding to an output or final fuel injection amount) Ti.
In the prior art, although the fuel injection amount is corrected using the correctional purged fuel amount currently calculated, the transport delay of the purged fuel vapors is not taken into account or considered in the fuel injection amount correction. In other words, the transport delay of the purged fuel vapors are not compensated in the prior art. Thus, the prior art is disadvantageous in that the accuracy in determining the purged fuel amount is not always satisfactory and hence, the accuracy in correcting the fuel injection amount is not always satisfactory.